Appropriate hoof preparation and shoe fit are integral to soundness and
performance. (6, 7, 10) Historically, inappropriate modification (trimming)
and extension (shoeing) of the hoof capsule have been considered the root
cause of foot pathology. (17, 41) Currently, diseases of the foot are considered
the predominant cause of equine lameness. (25)

The overall relationship between farriery and the ability of the limb (and
thus the horse) to absorb concussion and to function efficiently and safely
is compelling and obvious. A cohort study of 95 Thoroughbred horses concluded
that appropriate trimming and shoeing could reduce musculoskeletal disease
and enhance performance. (21) A case-control study of Thoroughbred horses
that were euthanized on California racetracks identified toe grabs on aluminum
racing plates as significant risk factors in fatal musculoskeletal injuries
associated with suspensory apparatus failures and metacarpal/metatarsal (cannon
bone) condylar fractures. (20)

Appropriate hoof balance requires an appreciation of the conformation and
athletic activity of the horse. Within the context of this paper, appropriate
hoof balance is defined as hoof preparation that enhances performance and
interferes minimally with long-term athletic ability. Stricter attention
to specific details of trimming and shoeing yields greater dividends for
the equine athlete due to increased musculoskeletal demands inherent to
performance. Important elements of foot preparation include the following:

hoof angle

hoof length

mediolateral hoof orientation

thickness of the sole, frog, and bar

wall contour and ground surface (3)

Shoes function as an important component of hoof balance by adding length
and weight to the digit. Shoes also alter the ground-contact area of the
hoof, its frictional characteristics, and its relative position to the axis
of the limb.

This review of hoof preparation, shoe fit, and shoe modification provides
specific guidelines that may be useful for performance horses. Conformation
and intended use are the final determinants for the appropriateness of specific
trimming and shoeing techniques for an individual horse.

HOOF PREPARATIONHoof angle

Hoof angle is the dorsal to solar angulation of the hoof measured at the
toe. Hooves are described as broken forward or club footed (high hoof angle)
when the angle of the hoof is too steep for its associated pastern; conversely,
hooves are described as broken backward (low hoof angle) when the angle of
hoof is too sloping for its associated pastern. (Fig. 1) (6) Classical and
modern renditions of hoof flight patterns portray a uniphasic (single-humped)
parabolic curve with the shape and often the length of the curve determined
by hoof angle (Fig. 2) (17, 39, 41, 45).

Fig. 1: Absence of dorsal alignment of the hoof
and pastern creates broken-forward and
broken-backward hooves.

Fig. 2: Hoof-flight patterns: Presumed and actual. Drawings A, B, and C are adapted from A. Dollar's
1898 classic depiction of the effects of hoof angle
alterations on hoof flight. These drawings were not
based on cinematographic studies. Unfortunately,
these quite inaccurate basic patterns are still displayed
in current textbooks. Drawing A represents the
presumed hoof flight that is produced by a normal hoof
angle and characterized by a uniphasic parabolic curve
with its maximal height centered on the parabolic curve. Drawings B and C display the presumed effects that
either lowering or raising the hoof angle would shift the
point of maximal height of the parabolic curve either
caudally or cranially, respectively. Cinematographic
and videographic studies of equine locomotion have
demonstrated definitively that hoof flight patterns are
biphasic; the first maximum which occurred shortly
after toe off is always higher than the second maximum,
which occurred shortly before initial ground contact. Drawing D represents seven limb positions in a stride
representing 6 horses trotting at 4 m/s and shod with
normal angle and hoof length. (Balch et al 1996)

Altering the angle of the hoof affects limb kinematics and kinetics through
predictable alterations in the suspensory apparatus (Fig. 3). Hyperextension
(excessive overextension) of the metacarpo (metatarso) phalangeal (fetlock)
joint is limited by the suspensory apparatus of the fetlock, a specialized
portion of the stay apparatus. The suspensory apparatus in the forelimb consists
of the following anatomic structures: the deep digital flexor tendon and
its associated accessory (inferior check) ligament; the superficial digital
flexor tendon and its associated accessory (superior check) ligament; the
suspensory ligament with its dorsal branches and its continuation the distal
sesamoidean ligaments; and the paired proximal sesamoids. The suspensory
apparatus of the hindlimb is functionally similar, though slight differences
exist anatomically. Collectively, the suspensory apparatus of both the forelimb
and hindlimb forms a ligamentous "hammock" which cradles the fetlock during
the stance phase. Energy is stored in the elastic suspensory apparatus while
the fetlock moves to its most distal and overextended position at the mid-stance
position. (16) After the limb moves beyond the mid-stance position, the release
of stored energy assists the flexion of the distal limb at lift off and the
elevation of the fetlock. (11) Logically, a change in the tension of one
of the three structures that comprise the suspensory apparatus should result
in a compensatory change in the tension of the other structures or a change
in the relative degree of overextension of the fetlock or both.

Fig. 3: Effects of hoof angle modifications on the joints,
tendons, and ligaments of the distal portion of the forelimb.
Raising the hoof angle flexes the distal interphalangeal joint,
flexes the proximal interphalangeal joint slightly, and extends
the metacarpophalangeal joint very slightly. Lowering the hoof
angle has the opposite effects on these joints. Raising the hoof
angle decreases tension in the deep digital flexor tendon and
increases tension in the superficial digital flexor tendon.
Lowering the hoof angle increases tension in the deep digital
flexor tendon. (Adapted with permission from Balch et al., 1995)

Altering the hoof angle changes the relative position of the flexor surface
of the distal phalanx and, subsequently, the tension of the deep digital
flexor tendon which inserts at that location. Not surprisingly, lowering
the hoof angle (either by trimming heel or adding a toe wedge) increased
maximal tension in that tendon in walking horses. (28, 40) Raising the hoof
angle (either by trimming the toe or adding a heel wedge) decreased maximal
tension. (28, 40, 46) Conversely, when the hoof angle was raised, the mean
maximal tension in the superficial digital flexor tendon and the suspensory
ligament increased. (46) However, the changes in the superficial digital
flexor tendon were only significant at the trot. (40, 46) Radiographic studies
have demonstrated that lowering the hoof angle extends the distal interphalangeal
(coffin) joint, extends the proximal interphalangeal (pastern) joint slightly,
and flexes the fetlock joint very slightly. Conversely, raising the hoof
angle produced the opposite effects on the joints of the distal limbs. (9)

Despite the popularity of the often-portrayed rendition of hoof flight as
an uniphasic parabolic curve influenced by hoof angle changes, a cinematographic
study (12) demonstrated that hoof flight was more accurately depicted as
a biphasic (double-humped) parabolic curve (Fig. 2) with the hoof angle exerting
little effect on shape or length of the hoof-flight pattern. Additionally,
low or high angles did not affect stride length in horses walking, trotting,
and cantering on a high-speed treadmill. (4) A low hoof angle slowed (increased)
breakover, the period between heel off and toe off; (4, 12) a high hoof angle
quickened (decreased) breakover. (4)

A hoof is trimmed appropriately when the dorsal surfaces of the hoof and
pastern are parallel. (3, 14, 38) Visual confirmation requires that horses
stand squarely on all limbs with cannon bones positioned vertically. Use
of a hoof protractor quantifies the hoof angle and facilitates record keeping
of the hoof angle of individual hooves over time. (Fig. 4) Successful use
of a protractor requires that the distal two-thirds of the hoof wall be dressed
with a rasp so that it is a straight continuation of the proximal third (without
dishes or flares).

Studies using instrumented shoes on horses moving on a treadmill at a walk,
trot, and canter determined that forces at the toe, medial heel, and lateral
heel were collectively the lowest when the hoof and pastern regions were
aligned. (5) This parallelism occurs for most horses when fore hooves are
trimmed between 50 and 54 degrees and hind hooves between 53 and 57 degrees.
(3) Conformation of the lower portion of the limb, especially the pastern,
may vary substantially with breed and individual. (9) Pastern variation may
also occur between paired limbs of a horse; horses afflicted with unilateral
chronic suspensory desmitis often experience contracture of the suspensory
apparatus which extends the fetlock joint and increases the slope of the
pastern in the affected limb. (6) The angle of the hoof should be related
to the conformation of that limb. (3)

HOOF LENGTH

Hoof length is the measurement of the length of the hoof wall. The hoof is
most commonly measured on the dorsum of the wall from the center of the toe
to the coronary rim, the most proximal portion of the wall. Careful palpation
of the coronet, the junction between the skin and the wall, reveals the most
proximal extent of hoof wall, which feels hard in contrast to the soft cutaneous
tissue covering the distal aspect of the pastern. The border between the
hoof and the hair on the pastern is approximately the same level as the coronary
rim and is readily recognized unless the hair at the coronet is allowed to
grow long. The use of calipers or rulers, either indirectly or directly,
provides quantitative measurements. (Fig. 4) Accurate measurements of hoof
length are important to owners and trainers since many breed and show
associations enforce restrictions on maximum hoof length. (27) For example,
the regulations of the American Horse Shows Association specify a maximum
toe length for horses competing in Arabian and Morgan divisions. (27) The
Horse Protection Regulations issued by the United States Department of
Agriculture to eliminate soaring abuses in Tennessee Walking Horses and racking
horses limit the height of pads to no more than half the length of the natural
hoof wall (Horse Protection Act of 1970).

Table 1 - Guidelines for
hoof length based on the weight of the horse

Horse Weight

Toe Length

Horse Size

Kg

Pounds

Cm

Inches

Small

360 - 400

800 - 900

7.6

3.0

Medium

425 - 475

950 - 1050

8.25

3.25

Large

525 - 575

1150 - 1250

8.9

3.5

These guidelines apply to most breeds of nongaited horses;
however exceptions occur. If the horse will be barefoot in a turnout environment,
an additional 0.5 to 0.6 centimeter (3/16 to 1/4 inch) of wall and a thicker
sole is advisable.

Lengthening the hoof increases the length of the lever arm over which the
hoof pivots at the end of the stance phase. A videographic study investigated
the effects of augmented hoof length (2.5 and 5.0 cm) in trotting horses.
(5) The effects were four-fold on the kinematics of the hoof. Lengthening
the hoof:

increased the maximum height of the hoof during the biphasic hoof flight

increased the vertical velocity of the hoof

shortened the horizontal distance between toe off and the achievement of
maximal height of the hoof

slowed breakover.

Despite traditional expectations of farriers and trainers, lengthening the
hoof did not affect stride length. (5) Lengthening the hoof amplifies torque
on the distal portion of the limb and increases pressure on the navicular
bursa by the deep digital flexor tendon at the end of the stance phase. This
increased length of the lever arm may lead to injuries of the suspensory
apparatus. (36) As the hoof grows and the toe moves away from the axis of
the coffin joint, the tendency of the horse to stumble, display awkward gaits,
and become injured by limb contact increases. (3)

Hoof length, either excessively short or long, may cause lameness. (6) When
trimming and shoeing short-footed (nongaited) horses, guidelines (Table 1)
for hoof length can be based on the weight of the animal, with the exception
of racing Standardbreds. (3) Racing Standardbreds are shod to maximize trotting
or pacing speed. Trotters generally require more carpal flexion (folding
action) than pacers and are often shod with longer hooves. (43) Horses of
other breeds that compete with an animated trot are shod similarly.

MEDIOLATERAL HOOF ORIENTATION

Mediolateral hoof orientation is determined by the relative lengths and angles
of the medial and lateral walls of the hoof capsule and is then modified
to achieve mediolateral balance for the limb. Two different techniques are
used to judge how to trim one side of the hoof relative to the other. (3)

The first technique, and the most commonly used, is a geometric
limb-axis-oriented procedure that prepares the ground surface of the wall
perpendicular to the axes of the cannon bone and phalanges. (Fig. 5) Variations
include the use of other anatomic structures as guidelines such as the hairline
at the coronet, the bulbs of the heels, the axis of the phalanges, or change
in medial or lateral orientation of the digit as it is manually flexed, extended,
and overextended at the fetlock.

Fig. 5: Attaining mediolateral balance through the
commonly used geometric-limb-axis-oriented
technique. Line A represents the axes of the
cannon bone and phalanges. The wedge labeled
C is the portion of the hoof necessary to be
trimmed if the solar surface of the hoof is to
be made perpendicular to line A.
(Adapted with permission from Balch et al 1991)

The second technique is result-directed and requires that hoof-landing patterns
be viewed. Commonly, the hooves are trimmed so that hooves land simultaneously
on the medial and lateral heels or land simultaneously on all segments of
the bearing surface of the hoof (flat-footed). The more the horse's limb
conformation approaches that of the ideal horse (conformation characterized
by limb straightness, proper angulation of joints, and a stance that is neither
base-wide or base-narrow), the more likely the two different techniques will
result in similarly trimmed hooves. Conversely, the more the horse's limb
exhibits conformational deviations, the less likely the two techniques will
result in similarly trimmed hooves. For example, if the mediolateral aspect
of the hooves of horses possessing a base-narrow, toe-out forelimb conformation
is trimmed perpendicular to the axes of the cannon bone and phalanges, the
hooves will commonly land lateral-heel first as opposed to flat-footed. (Fig.
6)

Fig. 6: Attaining mediolateral balance through
result-directed trimming. The left front hoof
lands first on the lateral aspect of the wall.
Result-directed trimming dictates that the
lateral side of the hoof be trimmed shorter
so that the medial and lateral heels land
simultaneously. (Adapted with permission
from Balch et al 1991)

Instrumented-shoe studies examining horses exercising on a high-speed treadmill
and measuring force on the toe, medial heel, and lateral heel characterized
weight distribution across the solar surface of the hoof during the stance
phase and hoof-landing patterns at various gaits. (5) Contrary to common
expectations, hooves that landed flat-footed or simultaneously on the medial
and lateral heels did not evenly distribute force across the solar surface
of the wall, but rather the medial side of the hoof was preferentially weighted
during the stance phase. The hoof-landing patterns were primarily lateral-heel
first in freshly trimmed horses that were judged visually by two experienced
farriers as landing simultaneously on both heels or flat-footed at a walk.
Instrumented shoes, measuring ground contact in milliseconds, identified
variations in hoof-landing patterns that were not distinguishable to unaided
human vision, because the events that defined the landing patterns occurred
at intervals shorter than the temporal resolution of human vision. Also,
the same study indicated that the landing patterns observed at a walk did
not necessarily reflect those landings at a trot or canter.

Achievement of appropriate mediolateral orientation is challenging in
well-conformed horses and may be very difficult in the performance horse
with conformational deviations. Which of the two general techniques,
geometric-limb-axis-oriented procedure or result-directed procedure, that
should be used in an individual horse is often controversial. In the sound
performance horse, a geometric limb-axis-oriented procedure using the least
distorted anatomy as a guideline is the most efficient technique. When the
horse is lame and the source of lameness is located within the digit, trimming
the solar surface of the hoof to achieve simultaneous landing of both heels
may effectively treat the lameness in many cases and in most cases is a useful
therapeutic supplement to more specific veterinary treatment. Traditionally,
lamenesses associated with inappropriate mediolateral balance and identified
as lateral distorted hooves, (17, 30) sheared heels, chronic heel soreness,
navicular syndrome, (32) and chronic metacarpophalangeal synovitis (2) have
been treated by trimming to achieve simultaneous landings of the medial and
lateral heel.

THICKNESS OF THE SOLE, FROG AND BAR

The solar surface of the hoof capsule consists of the distal projection of
the wall and horny layers of the sole, frog, and bars. These horny (epidermal)
layers cover and protect their associated coria (dermis) from trauma. Thicknesses
of these epidermal structures are difficult to measure directly in the intact
horse but can be estimated radiographically. Approximately 6 to 9 mm (1/4
to 3/8 inch) of horny sole covers sole corium in average-sized Quarter Horses;
(10) the sole is thinner in Thoroughbreds and Warmbloods.

Weight bearing on a limb affects sole descent and heel expansion during the
stance phase. As early as 1810, Clark correlated the flattening of the sole
concavity and abaxial expansion of the heels and concluded that interference
with normal heel expansion would be detrimental to soundness. (37) Experimental
studies done by Lungwitz (1891) and Dollar (1898) concluded that thickness
of the sole, as well as width of the hoof, affects sole descent and heel
expansion when the limb is weight bearing. The presence of the circumflex
artery and vein of the sole and a large palmar/planter (solar) venous plexus
sandwiched between the horny sole and the distal surface of the third phalanx
predisposes the foot to bruising if the sole is used for more than minimal
weight bearing. The insensitive sole acts primarily as a protective covering
for deeper sensitive tissue.

The frog has a limited weight-bearing function, although regular ground contact
is thought to stimulate normal frog growth and heel expansion. (3) Traditional
theories relate frog pressure to heel expansion. Accepted by many veterinarians
and farriers, the frog pressure/heel expansion tenet has been theorized as
ground pressure on the frog being distributed proximally into the digital
cushion and then passed abaxially through the hoof cartilages and venous
plexes to cause outward movement of the wall at the heels and quarters. (10)

Results of recent experimental studies suggest that the relationship between
frog pressure and heel expansion is more complex. Colles (1989) directly
evaluated the relationship between frog pressure and hoof wall expansion
using foil strain gauges attached to the wall. In this study, all horses
and ponies with normal frog pressure displayed heel expansion, however, reduced
frog pressure resulted in heel contraction in some horses and ponies, but
heel expansion in others. Enhanced frog pressure resulted in even less consistent
results with heel expansion in some horses, contraction in others, and in
some, expansion and contraction depending on the stage of weight bearing,
as well as lameness in some animals. In the opinion of the authors, periodic
weighting of the frog during normal locomotion stimulates frog growth and
promotes heel expansion in most horses; however, the geometry of the hoof
capsule plays a significant role in determining individual response.

Unshod horses regularly wear away the outer insensitive sole and bars as
those structures lose moisture, and becomes flaky. This normal exfoliation
is accompanied by distinct molting of the frog semi-annually. (3) Shoes diminish
the normal abrasiveness of the ground and subsequent thinning of these epidermal
structures. Superfluous sole, frog, and bars should be trimmed. The sole
should be thinned so that it is concave and not in the same plane as the
ground surface of the wall; ideally, the ground surface of the sole should
be trimmed to parallel the curved solar surface of the distal phalanx so
that uniform thickness is maintained. However, excessive thinning of the
sole predisposes the underlying sole dermis to bruising. The sole at the
toe can usually be thinned until glossy or "live" sole is visible. (10) Manual
palpation of the sole is useful in estimating its thickness; the degree of
springiness is related to sole thickness or distance to the underlying corium.
(3)

If the heels of the hoof are normally shaped and not contracted, the frog
may be trimmed level with the ground surface of the finished wall. Anaerobic
organisms (such as Fusobacterium necrophorum) are commonly found in the
collateral and central sulci of the frog, but rarely cause disease unless
trapped in air-deprived recesses. Removal of ragged edges and loose pieces
of frog decreases these pockets that may harbor the organisms causing thrush.
(10) The collateral sulci should be regularly cleaned and opened, especially
adjacent to the widest part of the frog, to allow escape of debris that collects
routinely in these natural recesses. The bars can be trimmed below the level
of the ground surface of the finished wall and contoured to match the concaved
adjacent sole. Excessive trimming of the bars is discouraged, as bars may
act as internal struts to inhibit heel contraction in the absence of normal
frog pressure. (3)

WALL CONTOUR AND GROUND SURFACE

Wall contour is the circumferential shape of the wall, and the ground surface
is the portion of the wall directly in contact with the ground. Wall contour
and ground surface partially determine the size of the hoof and structural
stability of the wall. Ground-contact surface of the hoof (or its extension
of the shoe) positions the weight-bearing surface of the limb in specific
spatial relationships to the rest of the limb. During the stance phase, the
fetlock acts as an articulation between the loaded fore (hind) limb and the
digit. The digit consists of a short link (the pastern formed by the first
and second phalanges) articulating with a supporting platform (the hoof enclosing
the distal phalanx) which is always positioned dorsally to force vectors
associated with the horse's weight and transmitted through cannon bones.
Both angular changes in the fetlock and deformation of the hoof are important
mechanisms for dissipation of concussive forces.

Hoof size reflects the physical dimensions of the enclosed distal phalanx
and is determined by the angle, contour, thickness, and length of the wall
at the toe, quarters, and heels, the prominence and shape of the frog and
bars, and the thickness of the epidermal sole and frog. Accurate assessment
of hoof size requires that the walls of hoof be dressed (rasped) so that
the distal two-thirds is a straight continuation of the proximal third of
the wall. A 1988 study of farriery management of navicular syndrome identified
the hoof characteristic of a small-foot-to-body-weight as statistically more
common in horses afflicted with navicular syndrome than a matched group of
sound horses. (48) Certain trimming and shoeing techniques have been incriminated
in the development of unnecessarily small feet. Shoes that are inadequately
sized for the hoof, nailing behind (palmar/plantar) to the widest part of
the hoof, and excessive trimming of the frog and bars hinder the physiological
expansion of the horse's heels during the stance phase. These techniques
promote heel contracture. In some horses and breeds, the hoof characteristic
of a small-foot-to-body size may be an inheritable trait and difficult to
modify. (48)

Slightly different configurations of the body, limbs, and hooves have been
described as ideal for dissipation of concussion and maintenance of soundness.
(1, 17, 19, 44) In 1903, Russell presented empirically derived guidelines
for identifying ideal limb-to-hoof conformation and specified with geometric
precision that the "normal center of equipoise" is located slightly caudal
to the apex of the frog. (41) Force-plate studies, measuring the horizontal
and vertical forces exerted by hooves and conducted 75 years later, confirmed
that the point of zero moment (location of a single vector representing all
the complex forces applied to the digit) was centered on the cranial third
of the frog at the walk and trot. (42)

Fig. 7: Relative position of the hoof in relationship
to the rest of the limb. Line B represents the "angle
of incidence" defined by the axes of the phalangeal
bones. Line C bisects the metacarpus (metatarsus) and
extends distally to brush the palmar (plantar) border
of the heels on the ground surface of the wall.
(illustration adapted from Russell, W. 1903)

Russell's drawings (Fig. 7) provide useful guidelines for the relative position
of the hoof (shoe) in relationship to the rest of the limb. The "normal center
of equipoise" is the termination of an "angle of incidence" passing distally
through the axes of the phalanges and identifies the center of the hoof.
Russell specified that the distance from this center point to any circumferential
position on the hoof (shoe) must be equal to secure a perfectly balanced
foot. Most importantly, he specified that an imaginary line passing distally
from the center of the fetlock joint should brush the most palmar (plantar)
extent of the ground surface of the wall. Further characterizing the normal
palmar (plantar) surface of the hoof wall of sound horses, Murray in 1873
stated that the palmar (plantar) face of the hoof wall should parallel the
dorsal face of the hoof wall.

The position of the hoof relative to the overall axis of the limb influences
trimming and shoeing the hoof and subsequent direction of hoof growth. It
becomes an especially important biomechanical consideration in horses with
angular limb deformities. When viewed from a cranial perspective, the
weight-bearing surface of a limb with angular deformity should be centered
under the fetlock as opposed to centered under the hoof capsule. (Fig. 8)

Fig. 8: Use of a shoe to extend the ground-bearing surface
of a limb
with angular deformities. Illustration A depicts a weanling with
multiple limb angular deformities: mild carpal valgus with lateral
rotation and severe fetlock varus with medial rotation. The
angular deformity of the fetlock displaces the force vectors
that are associated with the horse's weight laterally, relative
to the ground surface of the hoof; the upright, heavily loaded
lateral heel and the flare of the wall at the medial toe are
consequences. Illustration B uses red vertical lines adjacent
to the medial and lateral borders of the fetlock to suggest
the appropriate ground support for the limb. Illustrations C
and D depict the appropriately trimmed hoof and the
trimmed and shod hoof with a lateral extension, respectively.
As suggested by the contours of the fetlock, osteoarthritis
was confirmed radiographically. Shoeing with a lateral
extension was palliative, not curative.

The weight of the horse is transmitted through the skeleton to the distal
phalanx, which in turn is suspended from the inner surface of the hoof wall
by the stratum internum (the interdigitation of the epidermal and dermal
laminae). The hoof wall and, to a lesser extent, the bars, are well designed
for weight bearing as the stratum medium, the bulk of the hoof wall and bars,
is composed of hoof tubules. Hoof wall derives strength and shape from the
orderly proximodistal arrangement of tubular and intertubular horn. Strength
is compromised when hoof tubules in the toe and quarter regions bend abaxially
as they near the ground surface and create a dished or flared hoof wall.
In the normal horse, hoof tubules in the medial and lateral heels are straight
and parallel those in the dorsal hoof wall. If the palmar/plantar surface
of the hoof slopes more than that of the dorsal wall, then the load on the
most distal portion of the palmar/plantar hoof wall is increased. This increased
force may collapse the hoof wall. The palmar/plantar hoof wall continues
to lose its parallelism to the dorsal hoof wall as the tubular and intertubular
horn realigns itself to the ground surface.

This loss of parallelism causes diminished palmar (less commonly plantar)
ground support, is very common in performance horses, and is referred to
as underrun heels, sloping heels, (10) collapsed heels, (14) underslung heel,
(34) or run-under heels when the palmar face diverges at least 5 degrees
from dorsal face. (48) The presence of underrun heels may not be synonymous
with disease in the limb, but is a strong biomechanical predisposition to
injury. A 1988 study of 50 sound performance horses identified that 26 of
the horses had underrun heels. (48) The prevalence of underrun heels in racing
Thoroughbreds and Quarter Horses (35) supports the contention that this hoof
conformation may be an inherited characteristic that predisposes horses to
chronic heel soreness and other musculoskeletal injuries. The hoof tubules
in the portion of the heel that is underrun are bent forward, diminishing
their ability to resist compression. Concussion is disproportionately
concentrated in the heels due to the simultaneous reduction in ground surface
area and dorsal shift in location. This dorsal shift of ground-contact surface
hyperextends the coffin joint, increases tensile forces on the palmar (plantar)
aspect of the limb, and increases compressive loading of the dorsal surface
of the limb. (51)

When viewed from a cranial (caudal) perspective, the ground surface of the
hoof should be centered under the axis formed by radius and metacarpus (tibia
and metatarsus) in well-conformed horses. When viewed laterally, an imaginary
line that bisects the cannon bone should intersect the most palmar (plantar)
extent of the ground surface of the wall. The distal two thirds of the wall
at the toe, quarters, and heels should be a straight continuation of the
proximal third of the wall. The palmar (plantar) surface of the wall should
parallel the dorsal surface. Shoes function as continuations of hoof capsules.
Shoeing can extend the ground surface of the limb to compensate for ill-shaped
hooves (e.g., underrun heels) and poorly conformed limbs.

Part 1 references are at the end of part 2, which will appear in the
October issue of Anvil Magazine.)